1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to a Raman optical-fiber amplifier disposed between an optical transmitter and an optical receiver in an optical communication system.
2. Description of the Related Art
Recently, a higher demand for more data has forced the increase of transmission capacity in wavelength-division-multiplexing optical communication systems (WDM optical communication systems). One way to increase transmission capacity is to increase the number of transmission channels or the transmission rate. The transmission rate has now been improved a great deal and ranges from 2.5 Gb/s to 10 Gb/s, but more efforts are being made to further increase the transmission rate. Known methods for increasing the transmission capacity includes the parallel coupling of a conventional C-band erbium-doped fiber amplifier (C-band EDFA) with a L-band erbium-doped fiber amplifier (L-band EDFA), and the use of a new amplification medium, such as a tulium-doped fiber and Rare-earth doped fiber amplifiers, have disadvantages in that the available amplification band is narrow and the noise factor tends to be high. As a solution to these limitations, research efforts have been focusing on the Raman optical-fiber amplifier.
FIG. 1 shows the construction of a conventional Raman optical-fiber amplifier. The Raman amplifier comprises a first to a fourth isolator 120, 160, 180, 220; a first and a second pumping source 140 and 200; a first and a second wavelength-selective coupler 130 and 190; an erbium-doped fiber 150; a connector 170; and, a dispersion-compensation fiber 210.
The first isolator 120 permits optical signals inputted into the Raman optical-fiber amplifier to pass without filtration while shutting out backward light—i.e., light emitted from the first wavelength-selective coupler 130. The first wavelength-selective coupler 130 couples optical signals emitted from the first isolator 120 and pumps light with a 980 nm or 1,480 nm wavelength emitted from the first pumping source 140, then outputs them to be introduced into the erbium-doped fiber 150.
The first pumping source 140 forward pumps the first erbium-doped fiber 150 by exciting erbium ions. A laser diode that outputs pump light with a 980 nm or 1,480 nm wavelength can be used as the first pumping source 140. As such, the erbium-doped fiber 150 is forward pumped by pump light emitted from the first wavelength-selective coupler 130 and amplifies and outputs optical signals emitted from the first wavelength-selective coupler 130. The second isolator 160 permits optical signals emitted from the erbium-doped fiber 150 to pass without filtration while shutting out backward light.
The connector 170 serves to connect the erbium-doped fiber amplification portion 230 at the front stage thereof and the Raman optical-fiber amplification portion 240 at the back stage thereof—i.e., an optical fiber 110 connected with the erbium-doped fiber amplification portion 230 and an optical fiber 110 connected with the Raman optical-fiber amplification portion 240. The connector 170 is provided with a circular hall therein.
The third isolator 180 permits optical signals emitted from the connector 170 to pass without filtration while shutting out backward light. The second wavelength-selective coupler 190 couples optical signals emitted from the third isolator 180 and Raman pump light emitted from the second pumping source 200 then outputs them to be introduced into the dispersion-compensation fiber 210. The second pumping source 200 Raman-pumps the dispersion-compensation fiber 210. A laser diode that outputs Raman pump light with a 1,450 nm band wavelength can be used as the second pumping source 200. The fourth isolator 220 permits optical signals emitted from the connector 170 to pass without filtration while shutting out backward light.
As seen from the above description, the conventional optical-fiber amplifier comprises two amplification portions 230 and 240—i.e., the erbium-doped fiber amplification portion 230 at the front stage of the connector 170 and the Raman optical-fiber amplification portion 240 at the back stage of the connector 170. For this reason, price competitiveness is lowered due to the requirement of multiple optical components. In addition, the increased total volume causes poor integration.